QL> CREATE TABLE t1 AS SELECT ROWNUM ID FROM dual CONNECT BY ROWNUM<=10;數據庫
Table created
SQL> SELECT * FROM t1;
ID
----------
1
2
3
4
5
6
7
8
9
10
10 rows selected
SQL> CREATE TABLE t2 AS SELECT ROWNUM ID,'a'||ROWNUM NAME FROM dual CONNECT BY ROWNUM<=5;
Table created
SQL> SELECT * FROM t2;
ID NAME
---------- -----------------------------------------
1 a1
2 a2
3 a3
4 a4
5 a5
SQL> SELECT * FROM t1 LEFT OUTER JOIN t2 ON t1.id=t2.ID WHERE t1.id=4;
ID ID NAME
---------- ---------- -----------------------------------------
4 4 a4
SQL> SELECT * FROM t1 LEFT OUTER JOIN t2 ON t1.id=t2.ID WHERE t2.id=4;
ID ID NAME
---------- ---------- -----------------------------------------
4 4 a4
SQL> SELECT * FROM t1 LEFT OUTER JOIN t2 ON t1.id=t2.ID AND t1.id=4;
ID ID NAME
---------- ---------- -----------------------------------------
4 4 a4
3
8
9
6
10
7
2
1
5
10 rows selected
SQL> SELECT * FROM t1 LEFT OUTER JOIN t2 ON t1.id=t2.ID AND t2.id=4;
ID ID NAME
---------- ---------- -----------------------------------------
4 4 a4
5
8
3
1
2
10
6
7
9 spa
10 rows selectedorm
SQL> SELECT * FROM t1 , t2 WHERE t1.id=t2.ID(+) AND t2.id=4;
ID ID NAME
---------- ---------- -----------------------------------------
4 4 a4
SQL> SELECT * FROM t1 , t2 WHERE t1.id=t2.ID(+) AND t1.id=4;
ID ID NAME
---------- ---------- -----------------------------------------
4 4 a4ci
數據庫在經過鏈接兩張或多張表來返回記錄時,都會生成一張中間的臨時表,而後再將這張臨時表返回給用戶。io
在使用left jion時,on和where條件的區別以下:table
一、 on條件是在生成臨時表時使用的條件,它無論on中的條件是否爲真,都會返回左邊表中的記錄。form
二、where條件是在臨時表生成好後,再對臨時表進行過濾的條件。這時已經沒有left join的含義(必須返回左邊表的記錄)了,條件不爲真的就所有過濾掉。select
假設有兩張表:gc
表1:tab2數據
id
|
size
|
1
|
10
|
2
|
20
|
3
|
30
|
表2:tab2
size
|
name
|
10
|
AAA
|
20
|
BBB
|
20
|
CCC
|
兩條SQL:
一、select * form tab1 left join tab2 on (tab1.size = tab2.size) where tab2.name=’AAA’
二、select * form tab1 left join tab2 on (tab1.size = tab2.size and tab2.name=’AAA’)
第一條SQL的過程:
|
第二條SQL的過程:
|
其實以上結果的關鍵緣由就是left join,right join,full join的特殊性,無論on上的條件是否爲真都會返回left或right表中的記錄,full則具備left和right的特性的並集。 而inner jion沒這個特殊性,則條件放在on中和where中,返回的結果集是相同的。